Pseudomonas aeruginosa is a ubiquitous, opportunistic pathogen primarily infecting individuals with a compromised immune system, including transplant patients; severe burn patients, and those with cystic fibrosis (CF). In the context of CF, P. aeruginosa establishes a chronic condition whose morbidity and mortality results from lung damage. Due to the uncanny antibiotic resistance, CF patients infected with Pseudomonas often have chronic infections with limited therapeutic options. Therefore, for improved efficacy in treatment, a basic understanding of the pathogenic mechanisms utilized by this organism needs to be examined for possible therapeutic targets. While a majority of previous studies have focused on the initial stages of colonization and infection, we propose a new approach in searching for treatments. Mounting evidence indicates that a biofilm mode of growth is most likely involved in P. aeruginosa pathogenesis. Many virulence factors are produced by P. aeruginosa under this growth condition including alginate, rhamnolipids, type IV pili and cyanide. Others and we have discovered that a response regulator AlgR, controls all of these virulence factors. We discovered that AlgR is required for biofilm formation and that it represses the Rhl quorum sensing system in a biofilm. We have most recently created an algR allele that is constitutively active, algRD54E. PAO1 harboring algRD54E on the chromosome is defective in biofilm formation and does not produce pyocyanin. Our data show two new roles for AlgR: (i) AlgR is able to repress and activate genes when it is constitutively on and; (ii) AlgR is controlling the Rhl quorum sensing system of P. aeruginosa. The proposed research is innovative because we are examining the role of quorum sensing in mucoid P.a.. The proposed research is also innovative because we are determining the mechanism for a transcriptional regulator that regulates five different virulence factors and can activate transcription when it is either phosphorylated or unphosphorylated. The hypothesis to be tested is: AlgR controls the Rhl quorum sensing system in mucoid P.a., thus allowing the organism to persist in chronic infections. We will test this hypothesis with two specific aims: 1) We will determine the mechanism of AlgR control on the Rhl QS system in in vitro P.a. biofilm formation and 2) We will determine the mechanism of AlgR control on the Rhl QS system in P.a. virulence.